0 dict == clear
0 dict dup /x 1 put == clear
1 dict ==
1 dict dup /y 2 put == clear
2 dict ==
2 dict dup /z 2 put == clear

{
    { -1 dict } stopped {
        _errorname =
        stop
    } if
} stopped pop
_errorname =

<< >> == clear
<<>> dup /x 0 put == clear

<< /x 0 >> ==
<</x 0>> dup /y 1 put == clear

<< /x 0 /y 1 >> ==
<< /x 0 /y 1 >> dup /z 2 put == clear

{
    { >> } stopped {
               _errorname =
               stop
           } if
}  stopped pop

{
    { << /a >> } stopped {
        _errorname =
        stop
    } if
} stopped pop
_errorname =
